Cellular phones, personal digital assistants (PDAs), power-assisted bicycles, electric vehicles, hybrid vehicles, and other electrical devices and electromechanical apparatuses that are transportable without being left in one location have internal cells and other power storage devices so as to be capable of operating without being supplied by an external power supply. In many cases, charging of a power storage device is performed via a connection between a charge port provided to the device or apparatus and a power-supply device, interposed by a cable or the like. However, in recent years, there has been a demand for a technique for supplying electric power wirelessly without using such a cable, that is, without contact. In one technique for supplying electric power without contact, electromagnetic resonance coupling (referred to merely as “magnetic field resonance” below where appropriate) may be used. Magnetic field resonance is a technique in which a pair of resonance circuits having a natural frequency (resonance frequency) in common, e.g., a resonance circuit on the power-supplying equipment and a resonance circuit on the device or apparatus, are caused to resonate via a magnetic field, and electric power is transmitted via the magnetic field. Japanese Laid-open Patent Application No. 2011-234496 (Patent Document 1) discloses a technique for using magnetic field resonance to supply power without contact to a vehicle from a power source external to the vehicle.
However, when power is supplied via magnetic field resonance, a magnetic field produced around a coil unit including a resonance coil (antenna coil) provided to the resonance circuit, the resonance coil serving as an antenna, may produce electromagnetic noise. Electronic equipment or the like arranged around the coil unit may thereby be affected by the electromagnetic noise. For example, in power supply to a vehicle, it is possible for audible noise to be produced in on-board audio. Additionally, if a conductor made of metal or another such material is present within the magnetic field, it is possible for the conductor to be heated due to the electromagnetic noise. For example, when a coil unit is installed in the bottom part of a vehicle, metal components in the bottom part of the vehicle may be heated. Therefore, a technique for suppressing such electromagnetic noise is required. The coil unit disclosed in Patent Document 1 includes a plurality of resonance coils, the resonance coils being arranged so that the magnetic field produced in one resonance coil and the magnetic field produced in at least one other resonance coil are in opposite phases with regard to each other. This counterbalances the magnetic fields in areas beyond the resonance circuits and reduces magnetic field leakage (paragraphs 5-12, etc.). However, because the magnetic fields are counterbalanced, the strength of the magnetic field when the same electric power is imparted to the power-supply-side resonance coil is reduced, and the efficiency of power supply is also reduced. Because the resonance frequencies of the plurality of coils are set to the same value, harmonic noise is inadequately suppressed.
Japanese Laid-open Patent Application No. 2012-115069 (Patent Document 2) discloses an antenna coil configured as a composite coil including a flat coil and a loop coil arranged therein. According to Patent Document 2, the antenna coil is configured so that the magnetic flux generated by the flat coil can be counterbalanced by the loop coil. Patent Document 2 indicates that, when an electric current is passed through the flat coil, it is only when the frequency corresponding to the electric current matches the resonance frequency of the resonance circuit that includes the loop coil that the loop coil will be magnetized by electromagnetic induction, so that magnetic flux in a direction opposite that of the magnetic flux generated by the flat coil will be generated.
Specifically, in Patent Document 2, the resonance frequency of the resonance circuit that includes the noise-canceling loop coil is made to match the frequency of the noise to be canceled, whereby noise is reduced (paragraph 39, etc.). However, in, e.g., wireless power transmission by magnetic field resonance, a pronounced noise-reduction effect is not necessarily obtained even when the frequency of the noise source and the resonance frequency of the noise-canceling resonance circuit are made to match in this manner; this problem is becoming apparent in the experiments, etc. performed by the inventor(s).